Tidal spiral arms of Model 1hf. We show the time evolution between 0 and 1 Gyr (indicated in the panels of the first column) of the disc in the X − Y space, in density (left column), coloured by average radial velocity V_R (middle) and average azimuthal velocity (i.e. with respect to the mean at the given radius, third column). The dashed and dashed-dotted lines show the prediction of the spiral arms location following the winding with . The sector of the disc marked with dotted lines is a region whose dynamics we examine later in Fig. 6.